1. Department of Microbiology, University of Venda, Thohoyandou 0950, South Africa.
2. Department of Medical Virology, University of Limpopo, Pretoria 0204, South Africa.
3. AIDS Viruis Research Unit, National Institute of Communicable Diseases, Johannesburg 2131, South Africa.
4. Myles H. Thaler Center for AIDS and Human Retrovirus Research, University of Virginia, Charlottesville, Virginia 22908.